Synthesis gas is conventionally prepared by gasification, usually steam treatment, of coal or heavy petroleum fractions according to the reaction: EQU C+H.sub.2 O.fwdarw.CO+H.sub.2 ( 1)
accompanied, however, by side reactions forming carbon dioxide and small amounts of methane. When petroleum fractions are gasified the amount of hydrogen in the synthesis gas is higher than when coal is gasified. Some coal gasification processes involve the formation of higher amounts of methane, other hydrocarbons, tar, etc. During gasification oxygen is normally added in order to render the gasification self-supplying with heat.
By various reactions the synthesis gas may be converted into methane. In recent years such reactions have become increasingly important from the standpoint of preparing substitute natural gas (SNG), special gas transport systems and as a source of energy. Typical reactions include: EQU CO+3H.sub.2 .revreaction.CH.sub.4 +H.sub.2 O (2) EQU 2CO+2H.sub.2 .revreaction.CH.sub.4 +CO.sub.2. (3)
The carbon dioxide may also be converted with hydrogen into methane: EQU CO.sub.2 +4H.sub.2 .revreaction.CH.sub.4 +2H.sub.2 O. (4)
The so-called shift reaction causes an equilibrium between carbon monoxide and carbon dioxide: EQU CO+H.sub.2 O.revreaction.CO.sub.2 +H.sub.2. (5)
Moreover, synthesis gas may be converted by the Fischer-Tropsch synthesis (also called the FT synthesis) into methane and higher hydrocarbons, particularly paraffins and olefins, but possibly even into aromatic compounds: ##STR1##
The FT-synthesis is used for the production of motor fuel and other liquid fuels. It might also be of interest for preparing C.sub.2 -hydrocarbons but is not very suitable therefor because of its low selectivity. The C.sub.2 -olefin ethylene is a very expedient starting material for many organic syntheses so that petrochemical products thereby can be formed from lignite, coal and heavy petroleum fractions.
In contradistinction to the FT synthesis the invention especially aims at an efficient conversion of synthesis gas into C.sub.2 -hydrocarbons and in this connection it is observed that it is not essential whether ethane or ethylene is directly prepared because ethane may be cracked to ethylene at a high efficiency by well-known technology.
The FT synthesis is a kind of polymerization reaction in which the yield structure follows the so-called Flory distribution (see for instance G. Henrici-Olive et al, Angew. Chemie. 15, 136, 1976, and H. Schultz et al, Fuel Proc. Technol. 1, 31, 1977), a theoretical distribution of the various chain lengths which can be deduced mathematically from simplified kinetic assumptions. It can be shown that the Flory distribution theoretically may give a maximum yield of about 27% by weight of ethane and/or ethylene, calculated as the carbon in the hydrocarbons formed by the synthesis. In practice the yield of C.sub.2 -hydrocarbons in FT synthesis is almost always far below that expected according to the Flory distribution and only in a few cases has it been possible, under special circumstances, to obtain a C.sub.2 -hydrocarbon yield corresponding to or above that according to the Flory distribution. Moreover, it has not hitherto in FT syntheses been possible to avoid the formation of hydrocarbons having more than 4 carbon atoms.
Nearly all metals and to a considerable degree even oxides and hydroxides thereof have been proposed as catalysts for FT synthesis, frequently on support substances. There is often used one or more heavy metals with a promoter of an alkali metal oxide. The most important of the industrially employed FT catalyst metals are iron and cobalt. It is a drawback that they are also catalysts for the conversion of carbon monoxide into free carbon and carbon dioxide by the exothermal Boudouard reaction: EQU 2CO.fwdarw.C+CO.sub.2. (9)
The carbon formation causes irreversible damage to the catalyst and the reaction therefore imposes limitations on the usable process parameters. Moreover, the steam formed by the synthesis under some circumstances may cause the oxidation of iron catalysts, which totally or partly deactivate them. Other FT catalyst metals tolerate oxidation without concomittant deactivation. All known FT catalysts are more or less sensitive to sulphur poisoning and therefore the synthesis gas must be carefully rid of sulphur compounds before being subjected to FT synthesis. Many FT catalysts are sulphided but nevertheless are sensitive to sulphur poisoning; the sulphided catalysts containing only very small amounts of sulphur. The purification of the synthesis gas of sulphur compounds is a substantial economic burden on the FT process. In the majority of cases the sulphur content in the synthesis gas must be kept below 0.1 ppm, calculated as H.sub.2 S, whether it is to be methanated or used for FT synthesis. Dalla Betta et al (J. Catal. 37, 449, 1975) showed that 10 ppm of H.sub.2 S in the synthesis gas stream at 400.degree. C. destroyed Ru/Al.sub.2 O.sub.3,Ni/Al.sub.2 O.sub. 3 or Raney nickel catalysts.
Shultz et al (U.S. Dept. of the Interior, Bureau of Mines Report 6974, 1967) showed that ruthenium and molybdenum are promising catalysts for hydrocarbon synthesis whereas tungsten and noble metals other than ruthenium could be left out of consideration. Molybdenum, the catalytic activity of which is not on a par with that of the metals of the iron group, has since been investigated thoroughly and it is known that methanation and FT catalysts based on molybdenum are more resistant to sulphur poisoning than the metals of the iron group. Mills et al state (Catal. Rev. 8(2), 159-210, 1973) that catalysts of molybdenum oxides on Al.sub.2 O.sub.3 or other support had a rather high activity with respect to conversion of H.sub.2 /CO and a selectivity for methane formation of 80-94% and for C.sub.2 -hydrocarbon formation of 6-16% under certain circumstances. By sulphiding to molybdenum sulphides the activity decreased, which could be compensated for by pressure increase, and the yield of methane became about 94% and of C.sub.2 -hydrocarbons 5.9%. By the addition of H.sub.2 S to the synthesis feed gas the activity decreased (sulphur poisoning) and at the same time the selectivity changed with a drop in the methane yield at 64.6% and the C.sub.2 yield at 4.1% whereas the formation of C.sub.3 +C.sub.4 hydrocarbons increased at 29.4%. The effect of H.sub.2 S on the catalyst was reversible and temporary; i.e., its removal from the feed gas stream resulted in an increase in selectivity.
Madon and Shaw state in a review in Catal. Rev.-Sci. Eng. 15(1), 69-106 (1977) that FT catalysts based on metallic, oxidic or surface sulphided molybdenum do have decreased activity in the presence of H.sub.2 S in the synthesis gas but that the effect is temporary and reversible so that the original activity of the catalyst returns when the sulphur is removed from the feed gas; in this respect molybdenum contrasts strongly with nickel and ruthenium based catalysts in which the poisoning can be considered definitive and lasting because of the strong affinity of these catalysts to sulphur and because the chemisorbed sulphur is in equilibrium with very low concentrations of H.sub.2 S. Madon and Shaw also call attention to the fact that a catalyst based on molybdenum sulphides is strongly selective for methane formation (more than 90% of the carbon converted into hydrocarbons is converted into methane), whereas the presence of larger amounts of H.sub.2 S in the feed gas causes a change so that nearly 30% is converted into C.sub.3-4 hydrocarbons and only about 60% into methane. The amount of C.sub.2 -hydrocarbons produced is very small. From South Africa patent specification No. 766,137 it is known that vanadium-based catalysts for methane formation are rather sulphur resistant. Vanadium has a considerable selectivity for methane formation but it is stated in the said specification that by promotion of a V.sub.2 O.sub.3 -catalyst on a support of Al.sub.2 O.sub.3 with MoO.sub.3 a rather high yield of ethane can be obtained along with a decrease of the methane yields at concentrations of H.sub.2 S which are rather low but still much higher than those tolerated by nickel catalysts.
U.S. Pat. No. 4,151,190 relates to a process for optimizing the yield of saturated and unsaturated C.sub.2 -C.sub.4 -hydrocarbons. There is used a catalyst of 1-95% by weight of metal, oxide, or sulphide of Re, Ru, Pt or preferably Mo or W, 0.5-50% by weight of hydroxide, oxide, or salt of an alkali or alkaline earth metal and at least 1% support, preferably carbon or alumina. The alkaline component and the support enhance the formation of C.sub.2 -C.sub.4 -hydrocarbons and the Examples of the specification show that up to 40.5% of the hydrocarbons formed may be C.sub.2 -hydrocarbons. This result was obtained with a catalyst of tungsten trioxide and potassium oxide and a support of carbon. The Examples of the specification also show that even small amounts of gaseous sulphur compounds in the feed gas stream alter the selectivity of the catalyst in favor of a high methane formation and usually decrease its activity strongly. By removing the sulphur from the feed gas stream the original activity and selectivity may be recovered.
Accordingly, there is still a need in the art for a process and particularly a catalyst which, in Fischer-Tropsch syntheses, may give a high yield of ethane and/or ethylene and at the same time has a good activity in the presence of sulphur compounds in the synthesis gas thereby enabling the saving of the costs involved in sulphur removal.
It has now surprisingly been found that a small class of catalyst metals, viz. groups V-B and VI-B in the Periodic Table of Elements, in combination with metals of the iron group and supported on certain support materials is sulphur tolerant, retains a high degree of activity in the presence of sulphur and can give high yields of C.sub.2 -hydrocarbons.
Prior to describing the catalyst and process in detail, it should be mentioned that catalysts of a similar general type are known for various other purposes. Thus, Swedish patent specification No. 395,676 discloses a catalyst for the shift reaction (5) consisting of an alumina support impregnated with nickel and/or cobalt sulphide, aluminum sulphide and molybdenum sulphide. Swedish patent specification No. 407,680 discloses a process for the oxidation of methanol to formaldehyde using a catalyst obtained by the coprecipitation of dissolved molybdenum and iron compounds, admixing with titanium dioxide, and subsequent drying and calcination. U.S. Pat. No. 2,830,960 discloses a catalyst containing oxides of cobalt and molybdenum on activated alumina supports useful for hydrocatalytic desulphurization of hydrocarbons. U.S. Pat. No. 3,132,111 discloses a catalyst for such hydrotreating processes as hydrodesulfurization, hydrofinishing, and hydrocracking of normally liquid petroleum feedstocks. The catalyst consists of an alumina support containing a metal component of the iron transition group, metals from the fifth and sixth periods of group VI-B and vanadium, for instance a CoO.MoO.sub.3.Al.sub.2 O.sub.3 catalyst. U.S. Pat. No. 3,242,101 discloses a nickel-molybdenum-alumina hydrocarbon conversion catalyst, showing especially high activity for desulfurization, denitrogenation and hydrogenation of olefins and aromatics. Finally, U.S. Pat. No. 4,128,505 discloses a catalyst for hydrocarbon desulfurization, denitrogenation and aromatics saturation, which catalyst consists of coprecipitated titania and zirconia, the coprecipitate having associated therewith a mixture of (1) cobalt as metal, oxide or sulphide, and (2) molybdenum as oxide or sulphide.
Based on this background it is surprising that the process and catalysts described more fully hereinafter are active and highly selective for converting synthesis gas containing sulfur compounds into C.sub.2 -hydrocarbons.